SIMULATION OF A FIRE-SENSITIVE ECOLOGICAL THRESHOLD - A CASE-STUDY OFASHE JUNIPER ON THE EDWARDS PLATEAU OF TEXAS, USA

A model was developed to represent the establishment of a fire-sensiti ve woody species from seeds and subsequent survival and growth through five size classes. Simulations accurately represent structural change s associated with increased density and cover of the fire-sensitive As he jumper (Juniperus ashei, Buckholz) and provide substantial evidence for multiple steady states and ecological thresholds. Without fire, A she juniper increases and herbaceous biomass decreases at exponential rates until a dense-canopy woodland is formed after approximately 75 y ears. Maintenance of a grass-dominated community for 150 years require s cool-season fires at a return interval of less than 25 years. When i nitial cool-season fires are delayed or return intervals are increased , herbaceous biomass (fuel) decreases below a threshold and changes fr om grassland to woodland become irreversible. With warm-season fires, longer return intervals maintain grass dominance, and under extreme wa rm-season conditions even nearly closed-canopy stands can be opened wi th catastrophic wildfires.